Lead-free solder consists chiefly of tin and contains appropriate amount of copper, silver, nickel, bismuth, indium, phosphorus and germanium. Usually, a wetting phenomenon starts around a temperature range of 250° C. and therefore, a soldering process is performed by immersing members of a printed board and so on in a solder bath heated to the mentioned temperature range or contacting members of a printed board and so on with molten solder spouting formed in a solder bath.
However, copper used for a lead wire of a printed board, a member and so on is heated to the mentioned temperature range in the soldering process and elutes into solder. This phenomenon is so-called “copper leaching”. When copper leaching occurs, surface tension and fluidity of molten solder are affected as a copper concentration in a solder bath rapidly leads to raise a melting point of solder. As a result, a defect such as a solder bridge, a perforated solder, unsoldering, a cornuted solder, an icicle-formed solder and so on is caused leading to deterioration of solder quality.
Therefore, when a copper concentration in a solder bath rises, a part or a whole of solder in the solder bath are to be replaced. Used solder extracted in this process is discarded as it is or Sn is recovered by executing some additional process to reutilize as solder material.
As a recovering method of Sn, conventionally, a method utilizing melting point difference, an electrolytic refining method and so on are used.